This invention relates to antennas for the reception of electromagnetic energy in radio communication and more particularly to such apparatus that is to be used by submarines in a submerged state for the reception of energy in the E.L.F. (extremely low frequency) range, the upper limit of which may be regarded as 10 kilohertz. At such low frequencies the wavelengths within seawater are comparable in dimension to the length of the submarine so that the effect of the submarine on an associated antenna system cannot be ignored.
The desirability of long range radio communication in the E.L.F. band is predicated on the well known fact that the longer wavelengths more readily penetrate the ocean medium, are less affected by atmospheric conditions, and are less subject to natural and contrived interference.
Prior submarine antenna systems, such as described in U.S. Pat. Nos. 1,557,049 to Hammond; 2,840,700 to Browder; 3,229,295 to Watkin et al; 3,121,229 to A. Silverstein; and 3,372,395 to H. W. Kline are concerned with V.L.F. (very low frequency) reception, which term has been used in the past to include all frequencies below 30 kilohertz. In practice, however, the frequencies used were above the 10 kilohertz level which may now be considered to be the upper limit of the E.L.F. band. It has been found that the antenna systems and configurations which were effective for reception of the V.L.F. frequencies above 10 kilohertz are not well suited to reception in the lower frequencies now referred to as E.L.F.
It is desirable, as noted by the Browder patent, to have a submarine antenna system for submerged use which is as close as possible to being omni-directional in its capability. This has been accomplished, in V.L.F. systems for example, by utilizing a pair of spaced electrodes as antenna means to provide for reception from directions aligned with the electrode pairs while using a helically coiled conductor as "side-looking" antenna means. However, those systems which include helically coiled, side-looking antennas such as disclosed by the Silverstein and Kline patents, comprise loops or windings which are so structurally supported that all portions of the loops, turns, or windings must move as an entire unit. Movements in the earth's magnetic field will produce a voltage in each loop or turn which is exactly in phase with that in other turns. In the usual instances where the movements are oscillatory or vibratory in nature, e.g. from vibrations induced from the submarine's machinery or propellers, buffeting by passing water currents, and the like, there results noise voltages being produced which, in the case of E.L.F. systems, are highly degrading to good reception.